Gas Turbine Development Trends: Hydrogen Energy
Recent world trends related to the development of clean energy have led to an increased focus on the use of hydrogen as a cleaner fuel for gas turbines and with it, the need to develop gas turbine plants that can operate both on a mixture of hydrogen with natural gas and on pure hydrogen. The use of hydrogen as a fuel can significantly reduce COx emissions, but burning hydrogen with air increases the amount of nitrogen oxides NOx, therefore leading gas turbine manufacturers have made great efforts over the past decades to develop low NOx combustion technologies that can provide a high proportion of hydrogen content in the fuel, up to 100%.
In a modern gas turbine in a premixed combustor, operating conditions close to the lean-burn flammability limit are chosen to reduce oxides of nitrogen (NOx), where the lean-burn flammability limit is determined by whether or not a flame is ejected. The flame is blown off under the condition that the speed of the combustible mixture entering the combustion chamber is greater than the speed of the flame. The flame speed is highly dependent on the composition of the fuel, and in the case of hydrogen, the turbulent flame speed is known to be at least 10 times higher than that of a methane flame under gas turbine combustion chamber conditions due to its high diffusion and chemical reaction rate. In the case of gas turbine combustion chambers for power generation using natural gas, lean-burn combustion technology is mainly applied to reduce NOx (since NOx is exponentially dependent on the temperature in the combustion region), while gas turbines using fuel containing hydrogen (syngas ), are prone to flashback (flame speed is much higher than the speed of the incoming fuel mixture so that the flame moves back towards the entrance to the combustion chamber and nozzles). Previously, in such cases, combustion chambers without premixing were used to avoid the risk of damage and destruction of the nozzles and the entire system. In this case, a technique is applied that involves the injection of a large amount of steam or nitrogen to minimize the increase in NOx, but this, in turn, leads to a decrease in the temperature at the turbine inlet. Thus, for the latest hydrogen-fuelled gas turbines, leading manufacturers around the world have begun to develop special combustion technologies with pre-mixing or with special micro-mixers.
General Electric (General Electric)
General Electric has been developing gas turbine combustors for the last few decades to enable burning of fuels containing hydrogen [5, 6].SAC and MNQC (Figure 8) are essentially diffusion flame methods, and a large amount of NO x emission is inevitable. In order to reduce this, a large amount of nitrogen and steam is injected and operated. The DLN combustor has been commercialized, but its estimated hydrogen co-firing (15%), differs from its actual cofiring ability in operation (5%).
One of the last “Multi-tube mixer” (Figure 8) combustors was developed. This method compared to the swirl method makes it possible to pre-mix fuel-air in a short time and space, and it is said that it is advantageous in preventing backfire due to the high velocity at the nozzle exit.
Siemens designed/manufactured a combustor and nozzle by applying 3D printing technology for co-firing a high hydrogen fraction. Figure 9 introduces schematic diagrams of Siemens 3rd and 4th generation dry low emission (DLE) combustors developed for hydrogen-co-firing [5,7].
In both combustors Figure 9, two main and pilot fuel injection methods are used, and the mixture is introduced and premixed through various air passages. Siemens is taking a method of controlling the injection rate of air and fuel through each flow passage through a pre-test to optimize the flame position and combustion temperature according to the change in fuel composition.
Kawasaki Heavy Industries
In the conventional gas turbine combustion method, the flame field is widely distributed over the combustor. The micromixer combustor in the figure below replaces one large flame field with a very large number of small flames. NOx production not only depends on the temperature during the combustion reaction but is also linearly proportional to the residence time of the reactants in the high-temperature flame field. Accordingly, in the micromixer combustor, it is possible to significantly reduce NOx generation from the reduction of the residence time of this reactant. In addition, the risk of backfire can be eliminated due to the high-velocity flow jets from the very small (1 mm or less) nozzles at the mixer outlet (Figure 10). [5,8]
MHPS (Mitsubishi Hitachi Power Systems)
MHPS, a representative gas turbine manufacturer in Japan, has developed and operated various models of gas turbines for hydrogen combustion in various forms such as refinery gas, COG (coke oven gas) and BFG (blast furnace gas) in addition to syngas.
Figure 11 is a schematic diagram of MHPS for hydrogen-natural gas co-firing. As shown in the figure above, the existing MHPS premixed combustor for natural gas combustion adopts a strong swirl flow method. According to the development report of MHPS through a series of experiments, there was no problem of backfire when the existing combustor and nozzle were used up to the hydrogen level of 20%.
In addition, at the same time, development and research on a micro-grade gas turbine capable of 100% hydrogen combustion are being conducted, and this is named “Multi-cluster combustor”. The prototype and conceptual diagram of the combustor are shown in Figure 11. The combustion principle is similar to the micromixer method, replacing one large nozzle with numerous small diameter nozzles, increasing the air blowing rate to reduce the risk of backfire, and reducing the residence time at high temperature to generate NOx .
Power Systems Mfg., a subsidiary of Ansaldo Energia based in Italy, has developed the “FlameSheet™ ” combustor, a premixing system for hydrogen-natural gas co-firing , . What is unique is that this development program did not develop the entire gas turbine engine, but only the combustor, 9F, and Siemens/MHPS’ 501F, 501G, 701F, 701G, and Siemens’ 501B/D. Figure 12 shows the schematic diagram of the FlameSheet combustor and the velocity and temperature distribution using CFD [5,10].
The system is divided into two flow paths: pilot and main flow. As for the pilot flow, as shown in the radial direction (blue in Figure 12), the air coming in from the outside passes through the radial swirler, and at this time, it is premixed with the air sprayed from the swirler’s vane. The air-fuel mixture has a mechanism that enters the combustor inlet and is stabilized by swirl near the combustion chamber centerline. On the other hand, the main flow (red arrow) flows along the backside of the combustion chamber liner, passes through the main fuel injector, forms a fuel-air mixture, and flows into the combustion chamber after turning 180 degrees. In this process, the flame is stabilized at the desired position by the “aerodynamically trapped vortex”. The flame field formed by two different flow paths is separated from each other, and the flame is stabilized at the location according to the design intention by strong recirculation regions, and a more homogeneous premixer is supplied compared to the premixing system of other gas turbine manufacturers[5,10].
As we can see, the gas turbine has evolved substantially from the so-called smoke machines in the 17th century to the complex machines we’ve become familiar with today. With increased complexity comes increased design challenges, however, and while that is a topic for another blog (or 10), if you’re interested in learning more about modern tools to implement into your gas turbine project workflow, reach out to us by sending us an email at Sales@Softinway.com
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